Over the last two decades, almost every type of audio equipment turned to digital circuitry to improve and/or enhance performance. Available equipment to assist the hearing impaired (both severe and modest) includes both analog and digital, although digital approaches are gaining more and more ground because of size and circuit flexibility.
For typical loudspeakers, to render high quality audio, inherent variations with frequency in the amplitude, or sound level, of the sound reproduced by the speaker for a given level of signal driving the loudspeaker must be normalized. This process is known as speaker equalization. Traditionally, the design of equalizers has been performed by an experienced technician who uses precision instruments to measure the speaker characteristics and adjusts filters as needed to equalize the speaker. However, as will be appreciated, the speaker may be equalized without taking the environment into account if the speaker will be moved to several different environments, or there is no way to know what the environment might be or the “environment” changes. Alternatively, the speaker can be equalized to account not only for deficiencies of the speaker itself, but also the effects of the environment. In this way, the spectral performance of the loudspeaker is compensated so that for a given audio signal power level the amplitude of the resulting sound is approximately the same for all audio frequencies in the performance range of the loudspeaker. Such a procedure is manual, time-consuming and requires significant expertise but still does not necessarily yield the best equalization possible for the resources expended. In addition, the speaker equalization equipment often requires many cubic feet of space.
Hearing aids, although including an amplifier and a speaker, also include features which are at odds with the typical concert or public address loudspeaker. For example, whereas an amplifier and speaker system used for concerts and the performing arts demands extreme amplification and huge speakers, the smaller the size of the hearing aid the better. Also as was discussed above, where speaker equalization or a flat frequency response of the speaker or sound system is demanded for concerts and public performance, a properly tuned hearing aid does just the opposite. That is, the properly tuned hearing aid carefully avoids amplification of those frequencies at which an individual has acceptable or normal hearing while at the same time providing substantial amplification to frequencies at which the individual is impaired. It is, of course, possible that an individual's hearing loss could be the same across the audio spectrum in which case a hearing aid with a flat response might be desirable. Typically, however, hearing loss is frequency dependent, and for most individuals, the loss is progressively worse at frequencies at the high end of the audio spectrum. Although these differences result in opposite demands for a public sound system and an individual's hearing aid, much of the technical theory required to satisfy these opposite demands is the same. For example, whereas sound system equalization schemes may be used to compensate for sound power or volume output variations at different frequencies to obtain a flat sound system response, the same scheme may be used for sound system “unequalization.” That is, the scheme intentionally varies the power output of the hearing aid at various frequencies to achieve an output which is intentionally not equalized. This compensates for the wearer's impairment so that in most applications, the wearer perceives or hears as flat a response as is possible over the audio spectrum. However, as will be discussed hereinafter, a “hearing aid” may also be used by an individual with normal hearing for purposes of “enhancing” the individual's hearing ability above normal with respect to specific sounds, frequencies or environments. For these uses, the “hearing aid” will not strive to provide the user with a “flat” response over all frequency bands, but may instead intentionally peak the hearing ability at selected frequencies. Consequently, it will be appreciated by one skilled in the art that schemes and discussions related to equalization of individual sound systems are equally applicable to the “unequalization” required by individual hearing aids. Therefore, although much of the following discussion refers to equalization of sound systems by selective frequency dependant amplification, a person skilled in the art will recognize that the technology for achieving frequency dependent amplification to achieve “unequalization” as is necessary for tuning hearing aids is the same.
More recently, automated equalization schemes have been proposed. For example, one such proposed scheme is an automated graphic equalizer. Such an equalizer has a plurality of channels having fixed center frequencies and fixed Qs (ratio of center frequency to bandwidth of the channel) that cover the entire audio band with filters. It has been proposed to automate the equalization process with such an equalizer by using instruments to record the spectral behavior of a loudspeaker in an environment, and then, in an automated fashion apply to varying degrees such filters so as to compensate the loudspeaker performance and thus bring the resulting spectral behavior of the loudspeaker more closely to a target curve. The approach is limited in its capacity for optimization, and the equalizer is complex, making this approach impractical for widespread use in, e.g., low cost consumer audio products.
Another proposed scheme proposes equalizing a sound field by automatically deriving an inverse filter that is embodied in a combination of fast Fourier transforms (FFTs) and finite impulse response (FIR) filters. The inverse filter implementation is quite complex, however, requiring considerable computational resources, thus making this approach impractical also for widespread use in, e.g., consumer audio products. In addition, there is no provision for re-optimization in this scheme.
Therefore, it would be desirable to have a method and/or apparatus for the automatic equalization (i.e., tuning) of the speaker in a hearing aid that does not involve excessive complexity in implementation, such as for example, implementation costs, power consumption and size. It would also be desirable to have a method and/or apparatus for the automatic equalization or tuning of a hearing aid loudspeaker that automatically re-optimizes the equalization. The present invention provides such methods and apparatus.